| The LIM homeodomain (LIM-HD) family of transcription factors has been implicated in a wide variety of developmental processes, including early embryonic patterning, limb development, and neuronal differentiation. While the developmental functions controlled by LIM-HD proteins have been well described, the target genes regulated by these factors, and the precise mechanism by which these target genes are selected, remain largely unknown. We have focused on the Drosophila LIM-HD gene apterous (ap) which, among other things, plays a key role in wing development and axon guidance. Using a rescuing assay of the ap mutant phenotype, we demonstrate that the Ap LIM domains are essential for Ap function. LIM domains mediate protein-protein interactions, suggesting that Ap binds other transcription factors, and that these cofactors are necessary for Ap function. In a series of experiments, we demonstrate that the LIM-domain-binding protein Chip functions as an Ap co-factor. Chip binds Ap and is necessary for proper wing development and differentiation of Ap neurons. In addition to binding the Ap LIM domains, Chip interacts with all nuclear LIM domains tested, and also self-dimerizes. In this way, Chip links together two LIM-HD molecules, forming both homo- and hetero-dimeric complexes of LIM-HD transcription factors. Combinations of LIM HD genes have been shown to dictate axon pathfinding decisions, and we attempted to test the generality of this hypothesis by analyzing the Drosophila LIM-HD gene, dlim1. However, despite it's widespread expression, dLim1 does not overlap with any other LIM-HD member tested, and does not play a discernable role in axon pathfinding. dlim1 mutants die at late larval stages indicating an essential role for dlim1 in Drosophila development. Finally, to better understand mechanism of ap function in the wing disc, we rescued the ap phenotype with molecules known to be downstream of ap function. With only two molecules, Fringe and alphaPS1-integrin, we are able to restore wing structures to a remarkable extent. However, this wing consists entirely of ventral cell types, indicating that many aspects of wing development occur independently of dorsal identity, and that key functions of ap are mediated by a small number of downstream effectors. |
